Animate floating dust particles
I personally prefer post processing dust,  for a still image you would normally just find some free dust particles online and add them in via the compositor. But as for animated particles, I would Google some free dust asset animation packs and add them in the compositor using an image sequence node.

If can't find any online, you can always make your own! 


Build a cube and icosphere
Put icosphere on layer 2
Add a particle system to cube and


set emission from volume
change start and end values to 0
set it to emit the icosphere
disable emitter
mess with size and amount

add an emission shader to the icosphere
set the background to black
Set gravity to a really low value 
Add a couple of weak turbulence forces to push the particles around
After you animate this, you will have generated your own image sequence that you can composite onto any animation


Side note: if you really want to get fancy, you can use a noise texture set to camera mapping to control the strength of the emission node shader, then animate the x and y values on the mapping node to animate light going thorough the dust.